1. Place the dressing ingredients in a blender and blend until smooth.
2. Combine the chicken, water chestnuts, celery, grapes, and onions in a bowl.
3. Add l cup dressing and toss lightly to coat the salad well. Chill.
4. Cut each tomato at the stem end into eight equal wedges, leaving the bottom quarter of the tomato intact; pull segments apart gently.
5. Just before serving, fill the center of each tomato with the chicken filling.